Honeycomb material is a familiar product. It consists of an array of hexagonal cells made of flat sheet material and nesting so that each of the six walls of one hexagon is shared with a wall of an adjacent hexagon. When a honeycomb is made of stiff material it is very strong in the direction perpendicular with the axes of the hexagonal cells. It is frequently bonded between flat sheets to make strong but lightweight panels to make walls, airplanes, boats and other structures where rigidity, strength and light weight are important. Honeycomb material is also made of resilient material and in such form it has been used as a cushion. For example, U.S. Pat. No. 532,429 issued to Rogers discloses such a honeycomb structure as an insole.
The use of a honeycomb structure as a cushion is desirable because buckling of the thin walls of the honeycomb absorbs a great deal of energy per unit of thickness of the cushion. However, the honeycomb structure is inherently stiff and using a honeycomb cushion within a shoe causes the shoe to be inflexible.
One way known to manufacture a honeycomb structure is to place a number of ribbons side by side and bond them together intermittently. Thus, if two strips are bonded along their length along a given distance and then unbonded three times that distance, and if the other side of each strip is similarly bonded but with the bonding appropriately offset, expanding the elongated strips thus bonded in a lateral direction creates a honeycomb structure. This method of manufacturing a honeycomb structure will be discussed in greater detail hereinafter.
The difference between a honeycomb structure made by partial bonding of adjacent strips and conventional honeycomb structures is that one-third of the parallel walls of each hexagon are double, that is, are formed from the portions of two adjacent ribbons that were bonded together.
As stated above, the honeycomb structure made from intermittently bonded strips is created by laterally expanding the adjacent strips. However, the strips may also be overexpanded so that the two sides of the hexagon forming the top and bottom point straighten to lie in the same plane, in which case the hexagons become deformed into rectangles where two opposite sides are twice as long as the other two opposite sides. Overexpanded strips are very flexible in one direction and quite stiff in the other. The short sides of the rectangles are difficult to buckle and they are short and of double thickness, both of which contribute to stiffness. The long sides of the rectangle of an overexpanded honeycomb are twice as long as two short sides and therefore buckle more easily and in addition they are single thickness which also causes them to buckle more easily.